linux-kernel - Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

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Message-ID: <20150326095114.GB27751@pd.tnic>
Date:	Thu, 26 Mar 2015 10:51:14 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andy Lutomirski <luto@...capital.net>,
	Oleg Nesterov <oleg@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Alexei Starovoitov <ast@...mgrid.com>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

On Thu, Mar 26, 2015 at 10:37:32AM +0100, Ingo Molnar wrote:
> But here GAS generates the 10-byte opcode even if 'mov $0x12345678, 
> %rdi' is used, which is an unforced error.

7 bytes:

48 c7 c7 78 56 34 12    mov    $0x12345678,%rdi

which is REX + opcode C7 + ModRM + 4-byte immediate.

If it used the bX opcodes, it would have to use REX prefix too for the
%rdi register which would generate a 10-byte insn then:

               0:       48 bf 78 56 43 12 00    mov $0x12435678,%rdi
               7:       00 00 00

REX + opcode BF + 8 bytes immediate.

So I think using the REX register "rdi" forces it to use the REX prefix
and thus the longer instructions.

All IMHO of course.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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